1. Field of the Invention
The present invention relates to a variable frequency amplifier adapted to operate as tuned to each of operation frequencies of plural radio communication specifications and to reduce the strain of RF signals.
2. Description of the Related Technology
Radio equipment for use with radio communications systems typically exemplified by cellular phones, wireless LANs, Bluetooth and the like comprises an amplifier for amplifying received RF signals. The amplifier comprises: an amplifying element such as a field-effect transistor (FET); and a matching circuit at least disposed at an input side or an output side of the amplifying element. The matching circuit comprises: a capacitative element connected in series with a signal conductor; and a capacitative element connected in parallel with the signal conductor and with a ground. A power input side matching circuit is responsible for input impedance matching, whereas a power output side matching circuit is responsible for output impedance matching.
In the modern radio communications systems, so-called multiband or multimode radio communication equipment has been proposed which is adapted to switch between (among) plural radio communication specifications in order to overcome a problem associated with communication areas as well as to utilize the advantages of the individual specifications and to make up for shortcomings thereof. Such radio communication equipment adapted to the plural radio communication specifications comprises amplifiers corresponding to individual frequencies in order to tune to operation frequencies used in the individual radio communication specifications.
More recently, however, a demand for miniaturizing the radio communication equipment is increasing. As disclosed in JP-A-2005-079967 and JP-A-2000-114996, for example, an amplifier has been proposed wherein the capacitative element of the matching circuit is replaced by a variable capacity element, such as a variable capacitor or a varactor, so that the characteristics of the matching circuit may be conditioned according to the plural frequencies. The variable capacity element is varied in the electrostatic capacity thereof by applying thereto a bias voltage. This permits the dual-band or multimode radio communication equipment to employ a single amplifier so that the miniaturization of the radio communication equipment may be achieved.
In the current practice of radio communications, information is normally carried on signal amplitude. Namely, a normal RF signal is superimposed with a signal varied in amplitude, so that the RF signal is greatly varied in the amplitude. In a case where the matching circuit employs the variable capacity element the electrostatic capacity of which is varied by applying the bias voltage, the bias voltage is varied in association with the amplitudes of the RF signal. This causes the variations of electrostatic capacity, which result in strain.
The amplifier employing the variable capacity element, the electrostatic capacity of which is varied by applying the bias voltage, suffers greater deterioration of the strain characteristic as compared with a conventional amplifier employing a fixed capacitative element.
The invention is directed to a solution to such a problem, seeking to provide a variable frequency amplifier suffering less deterioration of the strain characteristic thereof and operable at plural frequencies.